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SAFIR: Testing the Coexistence of AGN and Star Formation Activity and the Nature of the Dusty Torus in the Local Universe.1

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SAFIR: Testing the Coexistence of AGN and Star Formation Activity and the Nature of the Dusty Torus in the Local Universe.1 Highlights of Spanish Astrophysics VII, Proceedings of the X Scientific Meeting of the Spanish Astronomical Society held on July 9 - 13, 2012, in Valencia, Spain. J. C. Guirado, L. M. Lara, V. Quilis, and J. Gorgas (eds.) SAFIR: testing the coexistence of AGN and star formation activity and the nature of the dusty torus in the local universe.1 M. S´anchez-Portal1;2, M. Castillo-Fraile1;2, C. Ramos Almeida3, P. Esquej4;5, A. Alonso-Herrero5, A. M. P´erezGarc´ıa3, J. Acosta-Pulido3, B. Altieri1, A. Bongiovanni 3, J. M. Castro-Cer´on1, J. Cepa3, D. Coia1, L. Conversi1, J. Fritz6, J. I. Gonz´alez-Serrano5, E. Hatziminaoglou7, M. Povi´c8, J. M. Rodr´ıguez Espinosa3 and I. Valtchanov1 1 European Space Astronomy Centre (ESAC)/ESA, Madrid, Spain 2 Ingenier´ıay Servicios Aeroespaciales, Madrid, Spain 3 Instituto de Astrof´ısica de Canarias, La Laguna, Tenerife, Spain 4 Centro de Astrobiolog´ıa,INTA-CSIC, Madrid, Spain 5 Instituto de F´ısicade Cantabria, CSIC-UC, Santander, Spain 6 Sterrenkundig Observatorium, Universeit Gent, Belgium 7 European Southern Observatory, Garching bei M¨unchen, Germany 8 Instituto de Astrof´ısica de Andaluc´ıa,Granada, Spain Abstract We present the Seyfert and star formation Activity in the Far-InfraRed (SAFIR) project, a small (15.1h) Herschel guaranteed time proposal performing PACS and SPIRE imaging of a small sample of nearby Seyfert galaxies. This project is aimed at studying the physical nature of the nuclear IR emission by means of multi-component spectral energy distribution (SED) fitting and the star formation properties of AGN hosts, as traced by cold dust. We summarize the results achieved so far and outline the on-going work. 1 Introduction The infrared (IR) range provides a unique probe into the coeval AGN and starburst phenom- ena. Most of the mid-IR (MIR) and far-IR (FIR) emission comes from dust, being of thermal arXiv:1210.7489v1 [astro-ph.CO] 28 Oct 2012 origin. According to the unified model, the central engine and broad-line region (BLR) are obscured by a thick dust torus. The grains absorb the UV/optical photons and re-radiate in the IR. The dusty torus emission peaks in the MIR (7-30 µm), being well characterized by existing facilities (eg. Spitzer , T-ReCS). It extends to the FIR, where the star formation (SF) related emission becomes dominant. So far this has been poorly constrained, due to the 1Herschel is an ESA space observatory with science instruments provided by European-led Principal In- vestigator consortia and with important participation from NASA. 2 SAFIR: testing AGN and star formation activity. limited spatial resolution and spectral coverage. According to [7], the MIR and FIR SED of Seyfert galaxies can be explained solely through thermal re-radiation of higher energy pho- tons by dust; this thermal emission is made up of three different components: (a) warm dust (120-170 K), heated by the AGN; (b) cold dust (40-70 K), heated by star formation, and (c) very cold dust (15-25 K), typical of dust heated by the general interstellar radiation field. Herschel [5] offers a unique window to study the FIR/sub-mm emission from nearby galaxies: on the one hand, the PACS [8] photometer permits imaging at 70, 100 and 160 µm bands with unprecedented spatial resolution (∼ 5.5 arcsec at 70 µm), thus allowing to characterize the SED minimizing the contamination from the host galaxy; and on the other, the SPIRE photometer allows to sample the formerly unexplored region in three bands at 250, 350 and 500 µm with a relatively high spatial resolution, probing the cold and very cold dust compo- nents across the galaxy and even in the nuclear and circum-nuclear region. These data can be used to build SEDs sampling the peak and Rayleigh-Jeans tail of the thermal cold/very cold dust emission, allowing to derive masses and temperatures. The star formation rate (SFR) can be also derived. The spatial sampling also allows distinguishing different regions within the galaxies (e.g. nucleus, arm, inter-arm regions). The two main subjects of study within SAFIR are: (a) The dusty torus: the current models fall in two main categories: smooth and clumpy distributions. When applying any AGN torus model to the actual SEDs of Seyfert galaxies, three ingredients should be considered: AGN, starburst and host galaxy. The starburst contribution is mainly determined by the FIR data points. Moreover, the torus and starburst emissions overlap smoothly in the FIR. Therefore, the availability of FIR data with high spatial resolution and wide spectral coverage is of crucial importance to break the model degeneracy. (b) Study of star formation and nuclear activity: the interrelationship between star formation and accretion onto massive black holes is crucial to understanding galaxy formation and evolution. Herschel provides a unique means to map the star formation activity across the host galaxies of AGN, allowing to to determine the location and extent of the (obscured) star forming regions and the variation of the SFR with the radial distance, and permitting an accurate mapping of the the dust composition and temperature. 2 Sample of galaxies and technical implementation The SAFIR sample comprises 18 nearby galaxies that were selected in such a way that different nuclear classes (Seyfert 1.x & Seyfert 2) were represented. Ten objects are barred spirals/lenticular galaxies. Five objects are peculiar/interacting systems. Four galaxies are confirmed Luminous or Ultra-luminous IR galaxies (LIRG/ULIRG). Only objects with either high-resolution ground-based MIR or Spitzer data were chosen. In addition, all the targets have optical, NIR, X-ray and radio data. Therefore, a complete multi-wavelength SED can be constructed for all targets. The observations were carried out using PACS and SPIRE scan map modes. The mapped areas for all targets were adjusted in such a way that the host galaxy and a back- ground region fit within the surveyed area. The 1σ sensitivities achieved by the PACS pho- tometer were approximately 3.6, 3.9 and 3.9 mJy/beam at 70, 100 and 160 µm, respectively. For the SPIRE photometer, 1σ sensitivities just above the extragalactic confusion limit were S´anchez-Portal et al. 3 achieved: 5.5, 7.6 and 6.4 mJy/beam at 250, 350 and 500 µm. These sensitivities allowed mapping, not only the nuclear and circum-nuclear regions but also large areas within the spiral discs. 3 Results In this section we present some already published results from the SAFIR project [9, 2, 1] and on-going work. 3.1 NGC 3081 NGC 3081 is an early-type barred spiral galaxy ((R)SAB0/a(r)), harboring a Seyfert 2 nu- cleus. A series of star-forming nested ring-like features are observed: nuclear (r1, 2.3 kpc), inner (r2, 11 kpc), outer (26.9 kpc). Our study combines Herschel FIR data with high- resolution ground-based NIR/MIR data [9]. The r2 ring is clearly resolved in our images up to 250 µm. The nuclear SED was computed with unresolved FIR fluxes (r ≤ 1.7 kpc) plus NIR and MIR data as shown in Fig. 1 (left panel). The torus was modeled using a clumpy model [4]. We wanted to test how the inclusion of FIR data affects the torus parameters, specially the torus size. We found that the inclusion of FIR data results in a notable increase of the +2 torus outer radius: Ro = 4−1 pc vs. Ro = 0.7 ± 0.3 pc obtained using only NIR and MIR data [10]. The radial distribution of clouds (as given by the clumpy model q parameter) also flat- tens when FIR data are included in the modeling (q = 0.2 vs q = 2.3). The remaining fitting parameters (width of the angular distribution, number of clouds, inclination angle and cloud optical depth) are in agreement with those obtained without the inclusion of the FIR range. At larger scales (1.7 kpc ≤ r ≤ 5.4 kpc), the FIR emission can be reproduced by cold dust at T = 28 ± 1 K (assuming a modified blackbody with emissivity β = 2), heated by young stars within the galaxy disc (likely located at r1). Finally, the FIR emission from the outer part of the galaxy is compatible with dust heated by the interstellar radiation field (T = 19 ± 3 K ). 3.2 Mrk 938 The morphologically peculiar galaxy Mrk 938 has been proposed to be the remnant of a gas-rich merger of two unequal mass galaxies [11]. According to its IR luminosity is a LIRG. The galaxy hosts a Seyfert 2 AGN and is known for a vigorous starburst activity. We have performed a multiwavelength study, including X-ray, NIR, MIR and Herschel FIR data [2]. The decomposition of the MIR Spitzer/IRS spectrum shows that the AGN bolometric con- tribution to the MIR and total IR luminosity is small [Lbol(AGN)/LIR ∼ 0.02], which agrees with previous estimations. We have characterized the physical nature of its strong IR emis- sion. The MIPS 24 µm and PACS 70 µm images reveal that the bulk of the star formation activity is located in a compact, obscured region of ≤ 2 kpc. We have used Herschel imaging data for the first time to constrain the cold dust emission with unprecedented accuracy. We have fitted the integrated IR SED (see Fig. 1, right panel) and derived the properties of the The SB–AGN connection in Mrk 938 193 data. Using the luminosities measured in Section 2.3 for the best- done using all the flux densities for λ 60 µmgiveninTable1,with ≥ fitting model to the X-ray data, we get a consistent value for the the normalization at 100 µm.
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